In recent years, the mounting density of electronic parts has been increased in accordance with the progress of electronic apparatuses, and new type of mounting methods, e.g., a method of mounting a semiconductor package having almost the same size to the size of a semiconductor chip, or mounting a bare chip, called chip scale package or chip size package (hereinafter, simply referred to as “CSP”), are being employed.
One of the most important characteristics for a mounted board having mounted thereon various electronic parts including a semiconductor element is reliability. In the reliability, interconnection reliability against thermal fatigue is directly responsible for the reliability of an apparatus using the mounted board and is therefore a very important issue.
One of the causes of lowering the interconnection reliability is a thermal stress due to the use of various materials having different coefficients of thermal expansion. Since the semiconductor element has a coefficient of thermal expansion as small as about 4 ppm/° C., whereas the wiring board on which electronic parts are mounted has a coefficient of thermal expansion as high as 15 ppm/° C. or more, a thermal strain is caused upon a thermal impact, which in turn generates a thermal stress.
A substrate having mounted thereon a semiconductor package having a conventional lead frame such as QFP and SOP, has kept reliability by absorbing a thermal stress at a portion of the lead frame.
However, in the bare chip mounting, a method of connecting an electrode of the semiconductor chip to a wiring board pad of the wiring board using a solder ball or a method of connecting by preparing a small projection called bump and using a conductive paste is employed. Therefore, a thermal stress is concentrated on this connecting portion thereby lowering the interconnection reliability. It has been known that introduction of a resin called underfil between the semiconductor element and the wiring board is effective for dispersing the thermal stress. However, the number of steps for mounting is increased, elevating the cost. On the other hand, there is a method of connecting an electrode of the semiconductor element to a wiring pad of the wiring board using conventional wire bonding, however, in this method, the board must be coated with a resin for encapsulation for protecting the wire, thus increasing the number of steps for mounting.
CSP can be mounted together with other electronic parts, and ones having various structures have been proposed as shown in Table 1 appearing at page 5 of “Future of CSP (fine pitch BGA) put into practical use”, the article of Surface Mounting Technology, March, 1997, published by Nikkan Kogyo Shimbun Ltd. Especially a method using a tape and a carrier substrate in a wiring board called interposer is being put into practical use. This includes systems shown in the above table that Tessera, Inc. and Texas Instruments Inc. are developing. In these systems, a semiconductor device is mounted through a wiring board called interposer, and hence excellent interconnection reliability is exhibited as reported in Shingaku Technical Report CPM96-121, ICD96-160 (1996-12) “Development of Tape BGA size CSP”, and Sharp Technical Journal, No. 66 (1996-12) “Development of Chip Size Package”.
Between the semiconductor element of CSP and the wiring board called interposer, an adhesive film is preferably used which lowers the thermal stress caused by the difference in coefficient of thermal expansion between the semiconductor device and the wiring board. In addition, the adhesive film is also required to have moisture resistance and endurance at high temperatures. Further, from the viewpoint of facilitating control of the production process, the adhesive film is desired to be of a film type.
An adhesive of a film type is used in flexible printed circuit boards, and those comprised mainly of an acrylonitrile-butadiene rubber are frequently used.
As printed circuit board materials for improving a moisture resistance, Japanese Provisional Patent Publication No. 243180/1985 discloses an adhesive comprising an acrylic resin, an epoxy resin, polyisocyanate, and an inorganic filler, and Japanese Provisional Patent Publication No. 138680/1986 discloses an adhesive comprising an acrylic resin, an epoxy resin, a compound having a urethane bond in the molecule and having primary amine at both terminals thereof and an inorganic filler.
A number of adhesives of a film type comprised mainly of an acrylonitrile-butadiene rubber are used. However, the adhesives have disadvantages in that the adhesive force is significantly lowered after treated at high temperatures for a long time and that a resistance to electrolytic corrosion is poor. The adhesives suffer marked deterioration especially in the moisture resistance test under severe conditions in, e.g., a pressure cooker test (PCT) treatment, which is used for the reliability evaluation of semiconductor relating parts.
The adhesive disclosed in each of Japanese Provisional Patent Publications No. 243180/1985 and No. 138680/1986 suffers marked deterioration when subjected to moisture resistance test under severe conditions in a PCT treatment.
The above adhesive cannot be used as a printed circuit board relating material in the mounting process of a semiconductor element on a wiring board, because the difference in coefficient of thermal expansion between the semiconductor element, and the wiring board called interposer is large, thereby generating a crack during reflow. In addition, the adhesive cannot be used, since it suffers marked deterioration when subjected to moisture resistance test under severe conditions in the temperature cycle test or PCT treatment.
The present inventors have found that, as described in Japanese Provisional Patent Publication No. 2000-154361, by reducing the elastic modulus of the adhesive film around room temperature, the thermal stress caused in the heating-cooling cycle due to the difference in coefficient of thermal expansion between the semiconductor chip and the wiring board can be lowered, so that no crack is caused during reflow and no damage is observed after temperature cycle test, thus giving the adhesive film excellent in heat resistance.
However, when demands on heat resistance and resistance to reflow crack become severer in future, it is necessary that higher level of heat resistance and moisture resistance be imparted to the adhesive film by increasing the peel strength at high temperatures and elastic modulus at high temperatures. Further, it is necessary to make an amount of volatilization from the adhesive a less level. When the amount of volatilization is at a certain level or higher, peripheral apparatuses are possibly contaminated during the process of operation.
An object of the present invention is to provide an adhesive composition which can form an adhesive film that has enough heat resistance and moisture resistance required for mounting semiconductor elements having a large coefficient of thermal expansion on a substrate for mounting semiconductor, and that can suppress an amount of volatilization at the time of use, a process for producing the same, an adhesive film using the adhesive composition, a substrate for mounting semiconductor, and a semiconductor device.